Need Help!- Y Axis Repeatability

[timprebleco]

Guy's
I have a Bridgeport Series II CNC Boss 6 mill. I have not ran it much since the spring when I purchased it, but every thing works. If I get into a verry heavy cut the Y axis does not repeat from one program cycle to the next. I have placed an indicator on the table, the mechanical counters seem to still be acurate but the machine does not always return to the original home postion when the cut is finished. I have done some engraving and PVC machining on it and for light weight work it seems to be fine.
I need to get some direction as where to go on this problem. Is it possibly in the stepper motor encoder? I thought about interchanging the motor with another axis.
When I bought it the guy said it had some acuracy problems. The prioce was right and none of the work I do is close tollerance work. But since it only seems to be one axis, I think I will try to fix it as apposed to upgrading the entire system at this time.
Thanks,
Tim

[machintek]

All of these older BOSS machines with steppers are OPEN LOOP (NO ENCODERS). Thus if the drives are not set up correctly (correct voltage 3 phase power, correct current, correct transistors, wires tight in terminals, etc) they may lose steps. The controls put out a certain amount of steps and expects the machine to go where it was told to go. There is also no backlash compensation. Thus is you develop play in the ball screw, you live with it or regrind/replace the ball screw. Also with age and time the magnetic fields in these motors starts to degrade. This also affects performance. The accel/decel that the motor can do. The life of the machine can be extended by detuning these accel/decel ramps on the ZCK board (RCK on BOSS 5 and older). Even the grade of way oil has an effect on a older machine.

George

[timprebleco]

George;
Thank you for the information. I am sorry I am taking so long to get back. I have tried slowing the feed down to reduce the vibration in heavy cuts and it becomes a lot more repeatable.
Since it only takes place in one axis, would maybe a motor replacement be in order? I have seen some used motors on E-Bay, but I may get one that is worse than the one I have. Does any one still make or rebuild these motors?
I have seen servo retrofits on the internet for these machines but I dont think I am ready to take on that size project.

[machintek]

The motors have permanent magnets in them With age, heat and vibration, they degrade. If you take a new stepper and turn it, you feel and hear a growl as the rotor turns in these fields. As it gets older, it turns more easily. But this means less torque will be put out by the motor. A used motor may be just as bad. Since this is only one axis, i would make sure it has the correct transistors, make sure the axis is getting 8 to 8.2 amps and that all the connections are good (no screw in fuse holders on the axis either).

George